The present invention relates to an implantable sensor and in particular to a sensor which can be implanted in a bone to be tracked by a tracking system.
Sensors which can be implanted into a body part of a patient can be used in surgical procedures for many applications. For example, implantable sensors can be used for measuring the temperature of a body part in which the sensor is located, and for measuring the amount of stress a body part is under. In particular, a sensor can be used to provide a registration mark whose location can be determined and tracked by a tracking system. Such a sensor can be implanted in a body part, such as a bone, so that the position of the bone can be tracked, e.g. during a surgical procedure.
When the sensor is used as a marker, a system in which the sensor is incorporated can be used to track the location of the sensor. This can be achieved using electromagnetic techniques. U.S. Pat. Nos. 5,391,199 and 5,443,489 provides details of systems which are applicable to the present invention, in which the coordinates of an intrabody probe are determined using one or more field transducers, such as a Hall effect device, coils, or other antennae carried on the probe. Such systems are used for generating location information regarding a medical probe or catheter. A sensor, such as a coil, is placed in the probe and generates signals in response to externally-applied magnetic fields. The magnetic fields are generated by magnetic field transducers, such as radiator coils, fixed to an external reference frame in known, mutually-spaced locations. Systems which are concerned with tracking a sensor in a three-dimensional space are also disclosed in WO-96/05768, U.S. Pat. No. 6,690,963 and US-A-2002/0065455. Subject matter that is disclosed in the specifications of the patents and patent applications referred to in this paragraph is incorporated in this specification for all purposes by these references.
Implantable sensors are typically implanted immediately prior to a surgical procedure and can subsequently be removed immediately after the completion of the procedure. An implantable sensor of this kind can have a threaded exterior surface, allowing the sensor to be implanted in the bone with a screwing action, and removed from the bone by the reverse unscrewing action.
It is important that such a sensor is securely held within the body part so as to prevent movement of the sensor once it has been implanted. Any movement of the sensor within the body part can be undesirable, especially when the sensor is used for tracking purposes, because any such movement can lead to inaccuracies in the tracking of the sensor and the body part in which it is implanted. Further, in order to minimise the duration of the medical procedure by which the sensor is implanted, it can be desirable that the sensor can be easily implanted and removed from its site in the body part.